In this article, we propose a novel detection method for underwater moving targets by detecting their extremely low frequency (ELF) emissions with inductive sensors. The ELF field source of the targets is modeled by a horizontal electric dipole at distances more than several times of the targets’ length. The formulas for the fields produced in air are derived with a three-layer model (air, seawater and seafloor) and are evaluated with a complementary numerical integration technique. A proof of concept measurement is presented. The ELF emissions from a surface ship were detected by inductive electronic and magnetic sensors as the ship was leaving a harbor. ELF signals are of substantial strength and have typical characteristic of harmonic line spectrum, and the fundamental frequency has a direct relationship with the ship’s speed. Due to the high sensitivity and low noise level of our sensors, it is capable of resolving weak ELF signals at long distance. In our experiment, a detection distance of 1300 m from the surface ship above the sea surface was realized, which shows that this method would be an appealing complement to the usual acoustic detection and magnetic anomaly detection capability.

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The performance of a three-axis magnetometer (TAM) is limited by various types of error; hence, the system must be calibrated prior to use. The convergence of calibration algorithms is quite sensitive to input data, and divergence yields incorrect parameter-estimation results when the maneuvers of TAM are constrained. Therefore, a calibration method, based primarily on improved truncated singular value decomposition, was proposed. The singular values were divided into groups; the small values were modified, whereas the large values were retained (without modification). Information with improved accuracy was obtained from these large values, while the less accurate information contained in the smaller values was modified, thereby avoiding divergent solutions. The experimental results revealed that the performance of TAM was significantly improved by the use of the proposed calibration method.

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